Current, high-quality mass-produced lacquer coatings for motor vehicles generally consist of an electrophoretically deposited primer which provides protection from corrosion and from stone impact, a subsequent primer surfacer coat which provides protection from stone impact and which smooths the surface, and of a two-layer coating which is applied subsequently for decorative purposes and which comprises a colour- and/or effect-imparting base lacquer coat and a protective clear lacquer coat which seals the surface. In practice, lacquer coating installations are often operated with a plurality of pigmented primer surfacers of different pigmentations, the colour shade of which is matched to that of the subsequent covering lacquer coating, and with a number of single-colour and effect base lacquers which determines the colour shade programme of the lacquer coating operation.
EP-A-0 265 363 describes a process for the prevention of yellowing of a multi-layer coating, wherein a primer in the form of a stoved cathodic dip lacquer, which contains special blocked polyisocyanates as crosslinking agents, is overcoated by the wet-into-wet method with a commercially available pigmented base lacquer and with a commercially available clear lacquer. The primer surfacer coat is omitted. Within the scope of the present invention, it has been shown that the direct overcoating of a stoved electro-dip lacquer coat with a two-layer coating comprising a customary solvent-based base lacquer, which is based on acrylic resins or polyester resins and which crosslinks with melamine resin, and a customary clear lacquer, results in a multi-layer coating which does not achieve the level of properties which is currently required for the original lacquer coatings of motor vehicles, particularly as regards the level of stone chip protection.
Aqueous coating media are known from DE-C-42 23 182 which contain polyester oligomer polyacrylates as binder vehicles and which are obtained by the copolymerisation, by a radical mechanism, of at least one hydroxy-functional ester of (meth)acrylic acid, at least one alpha,beta-ethylenically unsaturated carboxylic acid and optionally of one or more alpha,beta-ethylenically unsaturated monomers without hydroxyl groups and without carboxyl groups, in the presence of at least one hydroxy-functional polyester oligomer. These aqueous coating media are outstandingly suitable as aqueous clear lacquers, such as those which are used as covering lacquers for multi-layer lacquer coatings in the automobile industry. These coating media can also be used in pigment-containing form as colouring base lacquers which are overcoated with clear lacquers.
For reasons of efficiency and of achieving savings in materials, there is a desire in motor vehicle lacquer coating operations for a reduction of the number of operations and of the number of lacquer coats and for a reduction in the coat thickness of the lacquer structure, without incurring losses in the customary overall level of properties, however.